Various catalysts for a polymerization of a conjugated diene, such as 1,3-butadiene and isoprene, have conventionally been proposed, and some of them have been industrialized. For example, in production methods of a conjugated diene polymer having high cis-1,4 structures, combinations of a compound of a metal such as titanium, cobalt, nickel or neodymium with an organic aluminum compound are often used.
In addition, polymerizations of a conjugated diene using a catalyst containing a group 3 element of the periodic table are known, and various polymerization methods have hitherto been proposed. For example, Patent Literature 1 discloses a catalyst system including a salt of a rare earth metal, an organic metal compound of a group I to III element of the periodic table, and a fluorine-containing organic boron compound. Patent Literature 2 discloses a polymerization catalyst including a compound of a group IIIB metal of the periodic table, an ionic compound of a non-coordinating anion and a cation, and an organic metal compound of a group I to III element of the periodic table. Patent Literature 3 lists organic metal compounds including an element selected from groups 2, 12 and 13 of the periodic table. In addition, Patent Literatures 4 to 6 report polymerizations of a conjugated diene using a metallocene type gadolinium complex.
Among the catalysts described in Patent Literatures 1 to 3, however, ones showing their effects in Examples are mainly neodymium-based catalysts, and gadolinium compounds have not yet been made clear. In addition, the catalysts described in Patent Literatures 4 to 6 have defects that they have low catalytic activities of at most 540 g/mmol-Gd/hr.
A rubber composition containing a polybutadiene rubber (BR) or a styrene-butadiene rubber (SBR) as a main component, and including, in addition, a natural rubber, and the like has conventionally been industrially produced and used mainly as a material for a tire, a crawler for a crawler type traveling apparatus, an industrial rubber belt, or the like, utilizing its characteristic (Patent Literatures 7 and 8).
Requirements for reducing fuel consumption in automobiles and requirements for running safety on snow and ice have recently been increased for tire materials, and thus it has been desired to develop a rubber material having a low rolling resistance, (i.e., a high rebound resilience) and a large grip on a road surface such as a snow or ice surface (i.e., a high wet skid resistance). However, rubbers having a high rebound resilience such as a polybutadiene rubber (BR) are likely to have a lower wet skid resistance; whereas a styrene-butadiene rubber (SBR) has a defect in which the wet skid resistance is high, but the rolling resistance is also high. In order to solve the problems described above, various methods in which a low cis-diene-based rubber is chemically modified with a modifier in the presence of a lithium catalyst have been proposed, but the low cis-BR has an insufficient abrasion resistance, and this problem cannot be solved even by the modification. In addition, the SBR also has a low rebound resilience, and this defect cannot be solved even after the modification.
Patent Literature 9 discloses, as an example of a modification of a high cis-diene-based rubber, a method in which cis-1,4-polybutadiene is produced using a titanium compound having a cyclopentadienyl backbone as a catalyst, and then the product is modified by reacting it with 4,4′-bis(diethylamino)benzophenone, but the resulting product has a very small ratio of a weight average molecular weight (Mw) to a number average molecular weight (Mn), (Mw/Mn), of less than 1.5, thus resulting in a problem of processability.
A crawler type traveling apparatus, which is used in every field, such as a bulldozer, is configured by winding an endless belt-shaped elastic crawler around a drive sprocket, an idler, and multiple trunk rollers. It is often used in harsher conditions than those in wheels, and crawler structures have been improved in order to increase a running performance and durability. In addition, rubber compositions for a crawler, which are tougher, have a higher durability, and are excellent in low fuel consumption are required.
It is required, for a rubber composition used for a rubber belt, to have a high tensile strength, a moderate hardness, a softness, an elasticity, a high flexibility, and a high impact resistance, and further energy saving and weight saving are also required. In order to satisfy these requirements, reinforcement using an inorganic reinforcing agent or short fiber is proposed. In order to increase the hardness, tensile stress, breakage strength, abrasion resistance, and the like, however, if the amount is increased by addition of an inorganic reinforcing agent such as carbon black, then dynamic heat generation is increased, and a problem in which the weight saving is not attained is raised because of an increased specific gravity of the formulation. The rubber composition described in Patent Literature 8 has a problem in which the composition cannot necessarily obtain sufficient effects in the tensile stress and the elongation fatigue resistance, because microfine dispersion of 1,2-polybutadiene short fiber crystals in the butadiene rubber, which is a matrix component, is insufficient.
As for the polybutadiene, a vinyl⋅cis-polybutadiene in which syndiotactic 1,2-polybutadiene (hereinafter may sometimes be referred to as SPB) is dispersed in high cis-BR (hereinafter may sometimes be referred to as VCR) is known as a BR having higher functional characteristics, which utilizes the characteristics of the high cis-BR.
As the production method of VCR described above, for example, Patent Literatures 10 and 11 disclose methods for producing an SPB-containing high cis-BR composite using a cobalt catalyst. Patent Literature 12 discloses a method for producing an SPB-containing high cis-BR composite using a nickel catalyst.
In addition, Patent Literature 13 discloses a method for producing VCR in an inert organic solvent including mainly a C4 fraction such as n-butane, cis-2-butene, trans-2-butene, or butene-1. However, a method for producing VCR using a gadolinium catalyst has not yet been reported.